Production of a modified orthophotograph

ABSTRACT

A three-dimensional optical model is created from a stereo-pair of aerial photographs by projecting the photographs by means of two projectors, and a moving aperture scans the optical model. A photosensitive film is exposed to one of these beams and is shifted in a direction perpendicular to the plane of the film so that the portion of film that is being exposed intersects the optical model. Superimposed on the vertical shift motion of the film is a displacement tangential to the plane of the film, the amount of displacement being in a preselected relationship to the extent of the vertical shift, such preselected relationship preferably being a logarithmic one.

United States Patent Collins 51 May 16,1972

PRODUCTION OF A MODIFIED ORTHOPHOTOGRAPH Stanley Hoover Collins, Guelph,Ontario, Canada Canadian Patents and Development Limited, Ottawa,Ontario, Canada Filed: Sept. 15, 1970 Appl. No.: 72,487

inventor:

Assignee:

Related [1.8. Application Data Continuation-impart of Ser. No. 665,521,Sept. 5, 1967.

US. Cl ...355/22, 95/1 2.5, 355/52 Int. Cl ..G03b 27/44, 60% 27/68 Fieldof Search ..355/22, 77, 52; 353/120;

References Cited UNITED STATES PATENTS 3,191,170 6/1965 Lustig et al.....95/l2.5 X 3,486,820 12/1969 Blachut et al ..355/22 PrimaryExaminer-Samuel S. Matthews Assistant E.\'aminerRichard A. WintercornAttorney-Stevens, Davis, Miller & Mosher [5 7 ABSTRACT Athree-dimensional optical model is created from a stereopair of aerialphotographs by projecting the photographs by means of two projectors,and a moving aperture scans the optical model. A photosensitive film isexposed to one of these beams and is shifted in a directionperpendicular to the plane of the film so that the portion of film thatis being exposed intersects the optical model. Superimposed on thevertical shift motion of the film is a displacement tangential to theplane of the film, the amount of displacement being in a preselectedrelationship to the extent of the vertical shift, such preselectedrelationship preferably being a logarithmic one.

19 Claims, 17 Drawing Figures SHEET 3 BF 8 IN V ENTDR .rmmsy HOOVER emu;

PATENTEDMAY 15 I972 SHEET 8 BF 8 INVEUTDR 77M sy Hows? my PRODUCTION OFA MODIFIED ORTHOPHOTOGRAPH This application is a continuation-in-part ofU.S. application Ser. No. 665,521 filed Sept. 5, 1967 now U.S. Pat. No.3,602,592 issued Aug. 31, 1971.

This invention. relates to the production of photographic maps as usedfor cartography, photogrammetry and photo-interpretation, moreparticularly, the invention relates to a method and an apparatus forproducing a modified orthophotograph ,to form, along with an unmodifiedorthophotograph, a pair of orthophotographs that can be viewedstereoscopically.

The unmodified or true orthophotograph, as known in the prior art, iscreated from a stereo-pair of aerial photographs of a terrain. The termstereo-pair" is herein used to designate two photographs of the sameterrain taken at different angles, so as to be able to create athree-dimensional optical model of the terrain, and, as more fullyexplained below, includes a pair of photographs taken in the samedirection but at different distances from the terrain.

The optical model can be made to appear either in a suitable stereoscopeor and this applies to the present case at the area of intersection oftwo beams reflected from or transmitted through the pictures. Normallyfor the purposes under consideration, the two pictures are photographstaken sequentially from an aeroplane.

To obtain the aforesaid true orthophotograph, an apparatus is used inwhich the two stereo-mates that constitute the stereo-pair arerespectively placed in each of two projectors and are projected tocreate the optical model. Light filtering devices, such as complementarycolor filters or polarizers, are provided in the projectors, so that thesuperimposed images forming the optical model may be viewed throughsuitably colored or polarized glasses to enable an operator to see theoptical model in three dimensions. Alternatively, any means forpresenting the images individually to the two eyes of the operator, maybe used eg the so-called image alternator. Only a selected one of theaerial photographs is used for creating the orthophotograph, this beingdone by exposing a photosensitive film disposed on a table to the beamfrom the selected photograph, while a suitable color or polarizer filterprotects the film from exposure to the beam from the other projector.Only a small portion of the film is exposed at one time, the area ofthis film portion being defined by an aperture that scans the film. Theoperator shifts the table in a direction perpendicular to the plane ofthe film so that the film portion that is being exposed is always keptin intersection with the optical model.

The product of this procedure is the orthophotograph, that is,a'photographic map with full details of the terrain and every detailideally represented in its true horizontal position. Thus, anydistortion that exists in the selected original photograph, due to thefact that the terrain may not be entirely flat or that each portion ofthe photograph will not have been taken along a precisely vertical line,is corrected and it now becomes possible accurately to mosaic aplurality of orthophotographs containing adjacent sections of terrain.

Due to this method of producing the orthophotograph which is correctedby elimination of the distortions due to the different elevations ofparts of the terrain, the orthophotograph has the inherent disadvantagethat overall the terrain appears completely flat and, therefore, doesnot convey any information about the relative height of details.Evaluation of the orthophotograph is therefore restricted to horizontalmeasurements. Furthermore, any details, although still rendered inperspective, are difficult to identify or interpret.

To overcome the aforesaid disadvantage an apparatus has already beenproposed which produces a so-called droppedline chart. Such a chartnormally comprises a plurality of parallel lines each of which may haveseveral sections of different widths, the width of a section of linerepresenting the height of the corresponding detail over a datum plane.To produce the dropped-line chart, however, expensive and complicatedequipment is required, in addition to the apparatus for producing theorthophotograph. The dropped-line chart itself is produced on a separatesheet and further time-consuming manual procedure is necessary toachieve conventional contour lines from the dropped lines and then tointroduce the contour lines into the orthophotograph. Such contour linesare, of course, only an artificial representation of the terrain forms,and besides this fact they will, when directly drawn on theorthophotograph, hide many details. The procedure of drawing contourlines is particularly uneconomical in those cases where no permanent mapis required but only specific information, such as details of thedistances and slopes between two points, or determination of the area ofview or of the lines of sight from a given point.

It is the principal object of the present invention to produce amodified orthophotograph that, when viewed stereoscopically togetherwith a true or unmodified orthophotograph, will create athree-dimensional visual model of the terrain with both horizontal andvertical distances in predetermined scales.

According to the invention a method for'producing'the modifiedorthophotograph is provided, in which a displacement of the filmtangential to its plane is superimposed upon the above-mentionedperpendicular shift, this tangential displacement being related to thevertical shift in accordance with a predetermined function. Similar tothe prior art method, the combined motion of the film is so carried outthat at any instant of exposure the film portion that is being exposedintersects the optical model. The function interconnecting the twomotions of the film can be linear or not, and specifically it may varyin such a manner that the ratio between the displacement and the shiftincreases logarithmically with increasing values of the shift towardsthe projectors. Preferably, the exposure is performed by continuouslyscanning the film by an aperture moving in adjacent traces parallel to ascanning direction across the film, the tangential displacement of thefilm being a translation along this scanning direction.

In the foregoing method, the optical model is a real one produced at theplane of the film. As an alternative, the same effect can be achieved byproducing a virtual optical model in a binocular optical system.

As yet a further alternative to these types of visual models where anoperator must apply the vertical shift of the film manually to achievethe above-mentioned intersection, according to a further feature of thepresent invention, the modified orthophotograph can be producedcompletely automatically. In such a method, no longer is a model of theterrain generated. The diapositives are scanned by two beams emanatingfrom the spot of a CRT to obtain in the form of electrical signalsinformation concerning the parallax of image details. These signals areinterpreted by an analyzer to produce output signals that control afurther CRT by which a modified orthophotograph is made, such outputsignals containing the information for application of the equivalent ofthe above-mentioned tangential displacement.

It is a further object of the present invention to provide a device forproducing the modified orthophotograph. In a specific embodiment of theinvention, this device may comprise facilities for simultaneouslyproducing a true or unmodified orthophotograph as well as a modifiedorthophotograph. The unmodified orthophotograph is produced from onemate of the stereo-pair of aerial photographs and the modifiedorthophotograph from the other stereo-mate. Each one of such a pair oforthophotographs will hereinafter be referred to as a stereo-partner" tothe other.

The present invention will become apparent in more detail from thefollowing disclosure of examples of the invention described withreference to the accompanying drawings:

FIG. 1 is a perspective view of a prior art apparatus for producing anorthophotograph;

FIG. 2 is a perspective view of a portion of the apparatus according toFIG. 1, shown on an enlarged scale;

FIG. 3 diagrammatically shows the working principle of the prior artapparatus of FIG. 1;

FIG. 4 is a front view of a portion of the apparatus shown in FIG. 1,modified according to the present invention;

FIG. 4a is a fragment of FIG. 4 showing a modification;

FIG. 4b is a view of a further modification of FIGS. 4 and FIG. 5diagrammatically shows the working principle of the apparatus accordingto FIG. 4;

FIG. 6 is a front view, with parts broken away, of another embodiment ofthe present invention;

FIG. 7 is a fragmentary and somewhat diagrammatic side view taken alongthe line VIIVII in FIG. 6;

FIG. 8 is a front view of a further embodiment of the present invention;

FIGS. 9a-d diagrammatically illustrate different modes of operation ofthe apparatus according to FIGS. 4, 6 or 8;

FIG. 10 shows an unmodified orthophotograph and its modifiedstereo-partner;

FIG. 1 1 illustrates a terrain profile taken along the line XI- ,XI ofFIG. 10; and

FIG. 12 is a partly diagrammatic view of a further embodiment of theinvention.

PRIOR ART PROCEDURE FOR MAKING UNMODIFIED ORTHOPI-IOTOGRAPH (FIGS. 1 to3) The prior art device for producing an orthophotograph, as shown inFIG. 1, comprises, in general, a frame which is mounted on a table 21 bymeans of adjustable legs 22. At the top of the frame 20, there aremounted tracks 23 which carry a pair of optical projectors 24, 24'. Eachprojector carries a diapositive 25, 25', the images on the diapositivesforming a stereo-pair of aerial photographs with each other. Eachprojector 24, 24' is supported on a base member 26 which is adjustablealong the tracks 23, thereby providing variability of the distancebetween the two diapositives 25, 25 to enable them to be set inaccordance with the distance travelled by the camera between its takingthe two aerial photographs. Adjustment screws 27 permit a fineadjustment of the plane of each of the diapositives 25, 25 with respectto the table 21.

The table 21 carries a recording assembly, generally indicated at 28 andshown in more detail in FIG. 2. The assembly 28 comprises a base plate29 on the upper side of which a photosensitive layer or film 30 isprovided. Bars 31 of a U-shaped cross-section are fixed to two oppositeparallel edges of the base plate 29 and form two rails extending in alongitudinal direction which will hereinafter be referred to as theY-direction. A carriage 32, which is guided in the bars 31 along theY-direction, comprises a further pair of bars 33 each having a U-shapedcross-section and each extending in a direction which will hereinafterbe referred to as the X- or scanning direction and which isperpendicular to the Y- direction. The bars 33 guide a mask 34 having asmall rectangular aperture 35 in its center. Two cylindrical tubes 36are each connected to the mask 34 by means of a ball type universaljoint 37.

As shown in the overall view of FIG. 1, each of the tubes 36 forms theouter portion of a telescopic rod 38, the upper and inner portion 39 ofwhich is connected to a respective one of the projectors 24, 24' bymeans of another universal joint (not shown). Connected to the upperportion 39 of each telescopic rod 38 is a support arm 40 which extendsaround the associated projector 24, 24 and carries on its upper end aprojection lamp 41, 41'. If the mask 34 is moved across the film 30along the X- and Y-directions, each of the telescopic rods 38 remainsparallel to the beam of light that is emitted by the respectiveprojection lamp 41, 41' which beam extends through a specific small areaof the diapositive 25, 25, a projection lens disposed in the lower part42 of the projector 24, 24' and the aperture 35 in the mask 34, beingfocussed on the film 30.

The entire recording assembly 28 is shiftable in a vertical direction,which is hereinafter referred to as the Z-direction. To achieve thisshift, the upper end of a threaded spindle 43 is mounted on the bottomside of the base plate 29. This spindle 43 cooperates with a threadedsleeve 44 rotatably mounted on cross bars 45 of the frame of the table21 and controlled by a handwheel 48. It is important to the productionof accurate orthophotographs that the film 30 is shifted perpendicularlyto its own plane and that'the plane of the film 30, on which theorthophotograph is produced, is set parallel to a selected datum planein the optical model, corresponding to a given (usually horizontal)plane in the terrain of which the photographs were taken. To obtain thisresult, there are four guide rods 46 mounted on the comers of the baseplate 29 and extending downwardly therefrom along the Z-direction. Thefour guide rods 46 cooperate with four guide sleeves 47 connected to thetable 21.

In operation, the beams of light illuminating corresponding areas of thetwo diapositives 25, 25', create at their area of intersection athree-dimensional optical model. The beam of one of the projectionlamps, e.g. the projection lamp 41, passes through a red colored filterwhich may be disposed either in the lamp itself or in the lower part 42of the projector 24, while the other beam passes through a green coloredfilter. As a result, when the two focussed beams of light at theaperture 35 are viewed by an operator through spectacles having suitablyoriented red and green glasses, he is able to see the three-dimensionaloptical model there formed. While the mask 34 with the aperture 35 scansthe film 30, the operator shifts the recording assembly 28 vertically byrotating the hand wheel 48 so that the area of the film 30 that is beingexposed through the aperture 35 always intersects the optical model 51,as is diagrammatically shown in FIG. 3. It is to be understood that,while the two projectors 24, 24 are required to create thethree-dimensional optical model 51, the film 30 is only exposed to oneof the beams of light passing through either of the diapositives 25,25'. Therefore, the film 30 must be sensitive to the color of theselected beam (e.g. green) and insensitive to the color of the otherbeam (red).

A motor and drive unit 49, which is mounted on the recording assembly28, automatically performs the scanning and indexing of the mask 34across the film 30. The operation starts with the mask 34 at one end ofthe carriage 32 and with the carriage 32 at one end of the bars 31. Bymeans of cables 50 (FIG. 2) running within the channel of one of theU-shaped bars 33 and connected to the mask 34, the latter is moved inthe X- or scanning direction until it reaches the other end of thecarriage 32. At this moment the drive unit 49 indexes the carriage 32 inthe Y-direction by an amount equal to the length of the aperture 35 andreverses the motion of the mask 34, such indexing being performed bycable means (not shown) provided within the channel of one of theU-shaped bars 31, similar to the cable 50. The same procedure repeatswhen the mask 34 returns to the first end of the carriage 32. Instead ofscanning the film 30 in both senses of the X- direction, it is better toscan only in one sense and return the mask 34 without exposing the film,because of the apparent vertical shift that occurs, when the sense isreversed, and which shift introduces errors in the actual vertical shiftof the recordings assembly 28. While the film is exposed to therespective portions of the diapositives, the motion of the mask 34 istransferred to both of the projection lamps 41, 41 by means of thetelescopic rods 38.

It is important for the production of an accurate orthophotograph thatthe angle between each of the planes of the diapositives 25, 25' and theplane of the film 30 is the same as the momentary angle between thephotographing camera in the aeroplane and the assumed datum plane of theterrain, which datum plane will in most applications be taken ashorizontal.

The image produced on the film 30 in the above described apparatus is atrue or unmodified orthophotograph.

PROCEDURE FOR MAKING MODIFIED ORTHOPI-IOTOGRAPl-I FIG. 4 shows a detailof the same apparatus modified so as to produce a modifiedorthophotograph according to the present invention. In FIG. 4, a baseplate 29a carrying the film 30 is movably mounted on a mounting plate60. The bars 31 with the carriage 32 and the mask 34 are directlysupported on the mounting plate 60. Similar to the above described,construction, a threaded spindle 43 is fixed to the bottom side of themounting plate 60 and engages a threaded sleeve 44 which is mounted onthe cross bars 45 of the table structure and is rotatable by thehandwheel 48. Guide rods 46, which cooperate with guide sleeves 47 fixedto the table 21, serve to maintain orientation of the plate 60 of thismodified recording assembly 28a when the plate 60 is shifted in theZ-direction.

One edge of the base plate 29a projects in the X-direction and isprovided with a follower 61 that is freely rotatable about an axisextending in the Y-direction. The follower 61 engages a cam rail 62,which is disposed in a plane perpendicular to the Y-direction and has apreselected angle A with respect to the X-direction, by virtue of suchrail 62 being attached to the table 21 by means of a wing-bolt 63.Between the base plate 290 and the mounting plate 60, there are aplurality of rollers 64 retained in and spaced by a cage 65 so that eachof the rollers is rotatable about an axis parallel to the Y- direction.As a result, the base plate 29a is movable on the mounting plate 60along the X-direction.

In operation, the operator shifts the recording assembly 28a along theZ-direction by means of the hand wheel 48 so as continually to keep theportion of the film 30, that is being exposed through the aperture 35 inthe mask 34, intersecting the optical model. As the mounting plate 60 isshifted upwardly in the Z-direction, the follower 61 travels along thecam rail 62 and displaces the base plate 29a, relative to the mountingplate 60 to the left in the X-direction. In the embodiment according toFIG. 4 the bars 31 receiving the carriage 32 are connected to themounting plate 60 by means of straps 66 rather than to the base plate29a so that the horizontal displacement generated by the cam rail 62 istransmitted only to the base plate 29a and the film 30, while thescanning and indexing of the mask 34 is unaffected. As a result, themodified orthophotograph produced by such a device differs from theunmodified orthophotograph. produced by the above described prior artdevice in that every small portion exposed through the aperture of themask 34 is displaced in the X- direction by an amount depending on theelevation of the detail depicted in that small portion relative to thedatum plane selected in the terrain.

The X-direction along which the film 30 is horizontally displaced withrespect to the diapositives 25, 25 should be approximately parallel tothe image of the flight direction of the aeroplane in the optical model,i.e. the direction along which the aerial photographs have been takensequentially. Only with such a disposition will a stereoscopic imagepermit simultaneous stereoscopic vision of the overall terrain as wellas of every detail.

THEORETICAL CONSIDERATIONS (FIGS. 4, 4a, 4b, 5 and 9a to d) Therelationship between the shift of the recording assembly 28a in theZ-direction and the displacement of the base plate 29a with the film 30in the X-direction is determined by the angle A between the cam rail 62and the table 21 and can easily be varied by turning the cam rail aroundthe bolt 63. During the production of an individual modifiedorthophotograph, however, the setting of the cam rail 62 will bemaintained, and therefore, the parallax introduced for any points of theterrain will vary linearly with their respective elevations above orbelow the datum plane. With reference to FIG. 5, 55 is a profile sectionof the terrain taken along the X- direction with 53 indicating anupstanding (i.e. extending in the Z-direction) pole of a height dZ andlocated at an elevation Z above the datum plane 52. If the verticalshift of the film when scanning through the pole 53 is made equal to Zso that the portion on the film at which the pole 53 is to be depictedintersects the terrain 55 at the foot of the pole, the top of the polewill appear in the plane 30', in which the film lies, at a point 56 in apicture produced from the projector 24 and at a point 56' in a pictureproduced from the projector 24'. The distance between the points 56 and56' in these two pictures is the parallax d? for the top of the pole 53,which is determined for any given portion of the terrain by thedifferent angles at which the aerial photographs were taken, and whichis independent of any motion of the film. For reasons that are describedfully below in connection with FIGS. 9a to d, it is desirable that theparallax of the points of the terrain 55, which is artificiallyintroduced into the modified orthophotograph by horizontal displacementof the film, should match the above described inherent parallax of adetail height. In order to achieve this effect the angle A of the camrail 62 (FIG. 4) should be so selected that a point 54 on the terrainlocated at an elevation dZ above the foot of the pole 53 appearshorizontally olfset in the X-direction from its true horizontal distancefrom the pole by the value dP.

Based on these considerations, the required angle A for the cam rail 62is obtained from the expression cot A (dP/dZ) (B)/(H z where B thehorizontal distance between the projector lenses (along the X-directionH the vertical distance of the datum plane from the projector lenses;

Z the momentary reference distance of the film from the datum plane.

If the above equation is integrated, the result is P= B In (H/H- Z) (lnnatural logarithm) indicating that the relationship between the parallaxP and the distance Z is actually a logarithmic one rather than linear.

Assuming that the average flying height above the terrain is largecompared with the variations in elevation of the terrain, i.e. in thereduced model of FIG. 5 that H is large compared with Z, Z may beneglected in the above equations, and the relationship becomes a linearone, resulting in a straight cam rail 62 as shown in FIG. 4. In thosecases, however, where this assumption does not apply, particularly inmountainous terrain, and the resulting mechanical embodiment willrequire a cam rail 62a to be logarithmically curved accordingly (FIG.4a).

FIG. 4b shows a refinement of the structure of FIG. 4a. To enable theinstrument to be readily adaptable to different ranges of the assumedconstants B and H, a second cam rail 62b is arranged in series with thecam rail 62a, the rail 62b being linear and adjustable as to its angleC. This separate adjustability is necessary when the logarithmic rail62a is used, because to adjust the angle of the rail 62a itself wouldupset the correctness of the logarithmic function, rather than introducethe required linear variation into the constants of the equation. Therail 62a is shown as displacing horizontally a slider 67 that movesvertically with the mounting plate 60. The cam rail 62b is connected tomove with the slider 67 to generate a proportional vertical movement inan arm 68, which movement is transmitted as a horizontal one to the baseplate 29a. The mechanisms shown are merely intended to be illustrativeof the concept of applying a series arrangement of a logarithmicfunction and an adjustable linear function and obviously numerous otherarrangements can be used to achieve this result.

A further feature added to this embodiment is an adjustable height scale69 from which readings of the value of 2 can be obtained.

Adoption of the combination of a logarithmic function and an adjustablelinear function in accordance with FIG. 4b has the following practicaladvantages:

a. It permits the use of a single height indicator for measuring bothterrain elevation and the spot heights of individual objects.

b. The terrain model as seen in the orthophotographs appears smoothacross the scan lines.

c. Any errors of profiling produce no first order errors in themeasurements of heights of objects or of the terrain.

It is also possible to derive from FIG. the sense in which the film isto be displaced horizontally in the production of the modifiedorthophotograph. In the event that the unmodified orthophotograph istaken from the right hand projector 24' and the modified orthophotographfrom the left hand projector 24, the image point (such as 56) of adetail portion elevated above the terrain (as the top of the pole 53)will in the modified orthophotograph appear displaced to the right fromthe corresponding location of the image point (56') of the same detailportion in the unmodified orthophotograph. In order to obtainthe result,that a point in a higher area of the terrain (as point 54), whichappears at its true location in the unmodified orthophotograph, bedisplaced in the same sense, namely to the right, in the modifiedorthophotograph, the film itself on which the modified orthophotographis produced has to be shifted to the left.

FIGS. 9a to d illustrate the influence on the appearance of the terrain,when the unmodified and the modified orthophotographs are viewedstereoscopically, of the ratio of the horizontal displacement dP and thevertical shift d2 of the film on which the modified orthophotograph isproduced. FIG. 9a shows the true shape of a slope 90 extending in the Y-direction, i.e. in the direction perpendicular to the scanningdirection. If no horizontal displacement is applied, that is to say iftwo unmodified orthophotographs are produced, one from each mate of thestereo-pair of aerial photographs, and these two orthophotographs areviewed stereoscopically, the general terrain will appear flat, because,by definition, any

' given terrain point will appear at the same place in every trueorthophotograph. Thus two true orthophotographs cannot display therelative parallax needed to give an indication of a difference inelevation. Any details, however, smaller than the scanning aperture willbe seen in stereoscopic depth. Assuming that the center of the scanningaperture is maintained accurately to intersect the three-dimensionaloptical model, the stereoscopic appearance that results is a series ofinclined portions 91 (FIG. 9b) each parallel to the surface of theoptical model, i.e. each generally parallel to the corresponding portionof the slope 90. Each of the portions 91 belongs to a different scanningrow and has a length equal to the length L of the aperture 35 in theY-direction.

When a horizontal X-displacement is used, i.e. one of theorthophotographs is modified, a smooth stereoscopic representation 92 ofthe overall terrain according to FIG. 9c is obtainable, provided thatthe ratio d! to dZ matches the parallax of the detail. Any other ratiowill result in a stereoscopic terrain representation 93 (FIG. 9d) inwhich the overall slope is too shallow (or too steep) and which includesdiscontinuities 94 between the various portions of the overallrepresentation 93 i.e. between the scanning rows.

The exposing of the film 30 can be done step-by-step with the mask 34being moved relative to the film after each exposure by an amount equalto the width of the aperture 35. The preferred operational mode,however, is the continuous exposing mode in which the mask 34 is scannedalong the carriage 32 in the X-direction, and the carriage is indexed inthe Y-direction after each'completed scanning row.

ALTERNATIVE'PROCEDURES Because the film is displaced horizontally in theX-direction the speed of the aperture 35 relative to the film is notconstant and, therefore, the time of exposure of the difierent portionsof the film 30 varies with the elevation of the respective areas of theoptical model. Such variation can easily be compensated for bycontrolling the brightness of at least the selected one of theprojection lamps 41, 41' according to the horizontal displacement of thefilm 30; to this purpose, a potentiometer could be provided in the powersupply for the projection lamp or lamps, the slider of the potentiometerbeing coupled to the base plate 29a. If, on the other hand, thebrightness of the orthophotograph is not equalized, a further effect isobtained which may assist in interpretation when viewing the modifiedorthophotograph together with an unmodified one, since all slopes facingin one direction will appear brighter than the slopes facing in theopposite direction.

Instead of fixing the strap 66 to the mounting plate 60, as shown inFIG. 4, the strap can be secured to the base plate 29a. With such aconstruction, the bars 31, the carriage 32 and the mask 34 follow thehorizontal displacement of the base plate 29a. Provided that the motorand drive unit 49 is fixed to one of the bars 31, as shown in FIG. 8,the scanning motion of the aperture 35 relative to the film 30 is nowuniform and independent of the vertical shift of the recording assembly28a.

The image produced on the film 30 in the device shown in FIG. 4 is amodified orthophotograph according to the present invention.

As described above, the production of the modified orthophotograph canbe performed simultaneously with the scanning of the optical model.However it has already been proposed either manually or evenautomatically to scan the optical model in a first step, thereby storingthe entire terrain point-by-point with associated information about theX-, Y- and Z-coordinates of each point, such information being stored inany suitable recording medium, e.g. the memory of an analog computer.For producing the orthophotograph, the stored information is thentransmitted to corresponding drive circuits for exposing and scanningthe film on which the orthophotograph is to be produced. It is apparentthat such a process can easily be modified to produce a modifiedorthophotograph according to the invention. With a given relationshipbetween dP and til the computer can be programmed to modify theinformation about each X-coordinate by a value depending on theassociated Zcoordinate in accordance with the above equation.

SECOND EMBODIMENT (FIGS. 6 and 7) FIG. 6 shows a different embodiment ofthe apparatus according to the invention. In this apparatus thethree-dimensional optical model is not a real one which intersects theplane of the film 30 as described above; instead it is a virtual opticalmodel apparent only when the operator simultaneously views a pair ofimages by means of the binoculars 70 of an optical system 71 which isprovided in this embodiment. This optical system per se is the inventionof T. J. Blachut, G. W. Schut and A. J. Smialowski and is the subject ofU5. Pat. No. 3,486,820 issued Dec. 30, 1969.

According to FIG. 6 and FIG. 7 the optical system 71 consists of twonearly identical halves, each half corresponding to a respective one ofthe projectors 24, 24' and comprising essentially a half-transparentmirror 72, a Dove-prism 73 disposed in a tube 74 telescopic along theY-direction, further prisms 75 and 76 and a second tube 77 telescopicalong the X- direction and connected to each respective eye-piece of thebinoculars 70. In order to prevent rotation of the images visible in thebinoculars 70, each of the half-transparent mirrors 72 is rotatablymounted about an axis parallel to the X-axis, i.e. an axis perpendicularto the plane in which FIG. 7 is taken, but at half the angular speed oftravel of the aperture 35 in the scanning direction. Similarly, theDove-prism 73 is rotated about an axis parallel to the Y-axis, i.e. anaxis perpendicular to the plane in which FIG. 6 is taken, at half theangular speed of travel of the aperture 35 in the indexing direction.Each half-transparent mirror 72 splits the ray emanating from theprojector 24, 24 to produce a transmitted part of the light which isfocussed on the aperture 35 and a reflected part of the light whichproduces an image at a location within the tube 74. A similar image isformed at a corresponding location in the other part of the opticalsystem 71. These images are viewed stereoscopically by means of thebinocular pair of eyepieces 70 to produce a small portion of a virtualthree-dimensional optical model. The model appears in space, beingmovable along the tube 74 with rotation of the mirrors 72 that resultsfrom vertical shifting of the film 30. A diaphragm 78 containing ameasuring mark is disposed in each tube 74, and this mark also appearsstereoscopically to the operator, floating above or below the terrain.He moves the film 30 to move the optical model until the measuring markappears to lie in the plane of the detail of the optical model underconsideration.

, Since only the beam from the projector 24 is used to expose the film30, the half-transparent mirror 72 associated with the other projector24 may be replaced by a full mirror.

The main advantages of this second embodiment consist in a. that thearea of the virtual optical model to be observed appears always at thesame location in the apparatus without regard to any horizontal orvertical motion of the recording assembly 28a, since the binoculars 70are rigidly connected to the frame b. and that no colored filters orpolarizers are required, since only the beam of one of the projectors24' need be directed to the aperture 35.

STRUCTURE OF THE THIRD EMBODIMENT (FIG. 8)

In FIG. 8 an apparatus is shown which permits the simultaneousproduction of a true or unmodified orthophotograph as well as of amodified orthophotograph in one scanning and indexing operation. Such anarrangement will not only save time but also prevent false differencesin apparent terrain elevation due to the fact that the operator mayintroduce varying amounts of error in successive operations. Accordingto FIG. 8, a recording assembly 80 comprises a mounting plate 81 whichis shiftable in the vertical or Z-direction by means of a threadedspindle 43 engaging a threaded sleeve 44 and which carries two baseplates 82 and 83 in a side-by-side relationship along the X axis, eachof the base plates 82, 83 supporting a film 30a and 30m respectively.The base plate 82 is fixed to the mounting plate 81, while the baseplate 83 is supported so as to be displaceable in the X-direction bymeans of a number of juxtaposed rollers 64. A follower 61 provided atone end of the base plate 83 and engaging a cam rail 62 displaces thebase plate 83 in the X-direction, as the recording assembly 80 isshifted in the Z-direction. Two bars 31 having a U-shaped cross-sectionand extending parallel to the Y- direction are provided above both edgesof the mounting plate 81. In FIG. 8 the right hand bar 31 is secured tothe right hand edge of the fixed base plate 82, while the left hand bar31 is connected to the left hand edge of the mounting plate 81 by meansof a strap 66. A carriage 32 is movable in the Y- or indexing directionalong the bars 31, and within the carriage 32 a mask 84 is movable inthe X- or scanning direction. The indexing of the carriage 32 and thescanning of the mask 84 are performed automatically by means of a motorand drive unit 49 and suitable cable means (not shown). The parts of therecording assembly 80 so far described represent a combination of theprior art recording assembly 28 according to FIG. 2 and the recordingassembly 28a according to the invention as shown in FIG. 4.

Similar to the above described prior art apparatus, there are twoprojectors 85 and 85u disposed above the right hand film 3014. Twodiapositives 86 and 86a forming a stereo-pair of aerial photographs witheach other are placed in the projectors 85 and 85u respectively. Twotelescopic rods 87 and 87a control the scanning of the diapositives 86and 8614 by the projection lamps 88 and 88a, respectively, according tothe motion of the mask 84. A third projector 85m is provided above thefilm 30m which is supported on the horizontally movable base plate 83.The average distance between the center points of the films 30m and 30uis equal to the distance of two apertures 35m and 3514 provided in themask 84, and this latter distance is equal to the distance between thelenses of the two projectors 85m and 85. The diapositive 86m placed inthe third projector 85m is identical to the diapositive 86 and it isscanned by a projection lamp 88m controlled by a telescopic rod 87m. Itis important that the two projectors 85 and 85m with their diapositives86 and 86m are equally oriented and disposed at the same height relativeto the respective film planes.

OPERATION OF THE THIRD EMBODIMENT (FIG. 8)

An unmodified orthophotograph will be produced on the right hand film30a, and, simultaneously, a modified orthophotograph on the left handfilm 30m. As stated above, any error in the vertical shift of themounting plate 81 will apply to the modified as well as to theunmodified orthophotograph. It may be demonstrated that under thissimultaneous scanning small errors in the height at which the scan isperformed will not produce any apparent difference in the elevation ofthe terrain.

In the device shown in FIG. 8, the three-dimensional optical model isproduced by the projectors 85 and 8514 and will appear at a location tointersect the plane of the film 3014. As described above, the recordingassembly is shifted in the vertical direction so that at any instant ofexposure the area of the film that is being exposed through the aperture35a is kept in intersection with the three-dimensional optical model.For the exposure of the film 30u itself, only the beam passing throughthe diapositive 86a is used. Since the diapositive 86m is identical tothe diapositive 86 the same vertical shift must apply to the productionof the unmodified as well as the modified orthophotograph. It is,therefore, of importance that the films 3014 and 30m are maintained inthe same plane.

In order that, when viewing the resultant modified and unmodifiedorthophotographs stereoscopically, not only the overall terrain willappear three-dimensional, which fact is due to the horizontaldisplacement of the film 30m, but also the stereoscopic appearance ofthe details will be maintained, it is necessary that the unmodifiedorthophotograph be produced from one mate of the stereo-pair of aerialphotographs and the modified orthophotograph from the other stereo-mate.

Since both of the apertures 35a and 35m are provided in the same mask 84and are, therefore, moved at the same speed, the speed of the aperture35m relative to the film 30m varies with the horizontal displacement ofthe base plate 83, and the above described means for controlling thebrightness of the projection lamps may be provided.

It is also possible to equip the device shown in FIG. 8 with the opticalsystem 71 used in the arrangement of FIGS. 6 and 7 It is furthermorepossible to operate the device of FIG. 8 having only two projectors 85and 8514. Suitable further optical means will then be necessary. Forexample, a half transparent mirror can be disposed in the ray emanatingfrom the projection lamp 88 such that the transmitted part of said raypasses through the aperture 35a, while the reflected part is rereflectedin a direction parallel to the transmitted part and so as to passthrough the aperture 35m. Such a device has the inherent advantage thatno differences between the two diapositives 86 and 86m, nor betweentheir disposition relative to the respective films, can occur.

FOURTH EMBODIMENT (FIG. 12)

In Canadian Pat. No. 729,857 granted Mar. 15, 1966 to Gilbert L.Hobrough an automatic stereoplotting system is described which may bemodified and used for carrying out the present invention. The modifiedsystem is illustrated in FIG. 12. To a certain extent, the workingprinciple of this system can be considered an inversion of the principleof the above described manual scanning systems. As before, a stereo-pairof aerial photographs of the terrain is provided in the form of twodiapositives 125, 125' disposed in a side-byside relationship along theX-direction and in the same plane, which plane is parallel to the screen124 of a cathode ray tube 120. The above described three-dimensionaloptical model of the terrain no longer exists as a real and visual one,but for a better understanding of this embodiment, one has to imagine anotational three-dimensional model at the screen 124 of the CRT 120,with the screen being disposed in a plane corresponding to the plane ofthe prior photosensitive film. The Z-shift of this plane is achieved bya vertical shift of the CRT 120.

A first ray 121 emanating from the light spot on the screen 124 of thecathode ray tube 120 is directed by suitable focussing means (not shown)onto a first photo-sensitive cell 122 after passing through thediapositive 125, and a second ray 121 emanating from the same light spotis similarly directed onto a second photo-sensitive cell 122' afterpassing through the other diapositive 125'. Instead of using two raysfrom one light spot, two CRTs could be provided with all their inputsconnected in parallel, a first ray emanating from one CRT projected tothe diapositive 125 and a second ray emanating from the other CRTprojected to the diapositive 125. The direction of the rays 121, 121 andthe disposition of the diapositives 125, 125 are so that correspondingareas of both diapositives 125, 125' are always scanned simultaneously.The output signals from the cells 122, 122 are fed into an X-parallaxanalyzer 130 which interprets any parallax between the two images ofeach detail represented by the input signals, such parallax providing anindication of the face that the detail depicted in the area that isbeing scanned is elevated above a datum plane. The X-parallax analyzer130 which interprets any parallax between the two images of each detailrepresented by the input signals, such parallax providing an indicationof the fact that the detail depicted in the area that is being scannedis elevated above a datum plane. The X- parallax analyzer 130 providesan output signal having an amplitude according to the amount of theparallax, which output signal is supplied via an amplifier 131 to adrive unit 132. The drive unit 132 shifts the CRT along the Z-directionby an amount such that the parallax detected by the analyzer 130disappears. The scanning of the lightspot across the screen 124 of theCRT 120, and thereby the scanning of the beams 121, 121' across thediapositives 125, 125' is achieved by deflecting the cathode ray bymeans of a pair of X-deflection coils 133, as well as a pair ofY-deflection coils 134. The X- deflection coils 133 are connected to anamplifier 135 which in turn is coupled to an X-deflection signalgenerator 136; the Y-deflection coils 134 are connected to an amplifier137 which in turn is coupled to a Y-deflection signal generator 138.

A second cathode ray tube 140 is provided to produce on its screen theimage of a modified orthophotograph which image is projected by means ofa suitable lens system (not shown) onto a photo-sensitive film 141 toform there the modified orthophotograph according to the presentinvention. The intensity of the cathode ray of the CRT 140 is controlledby the output of an amplifier 142 which receives an input signal fromeither of the photosensitive cells 122, 122', and which cathode ray isdeflected by means of a pair of X-defiection coils 143, as well as apair of Y-deflection coils 144. The Y-defiection coils 144 are connectedto an amplifier 145 which in turn is coupled to the same Y-deflectionsignal generator 138 that is connected to the Y-deflection coils 134 ofthe CRT 120. As a result, the Y-deflection of the cathode ray of the CRT140 is in phase with that of the CRT 120 and both Y-deflections are ofsimilar amplitudes. The X-deflection coils 143 of the CRT 140 areconnected to an amplifier 146, which is coupled to a modifier 147. Oneinput of the modifier 147 is connected to the output of the X-parallaxanalyzer 130, while the other input of the modifier 147 is coupled tothe same deflection signal generator 136 that is connected to theX-deflection system of the CRT 120. Again, the connection of theX-deflection systems of both CRT's to a common X-deflection signalgenerator provides synchronous scanning along the X- direction of thediapositives 125, 125' as well as of the film 141, and, disregarding theeffect of the modifier 147, both cathode rays are deflected along theX-direction by similar extents. The function of the modifier 147 is toadd to the X- deflection signal created by the generator 136 a signalthat is obtained by multiplying the output signal from the X-parallaxanalyzer, i.e. the signal effecting the Z-shift of the CRT 120, by avalue corresponding determined from the above equations.

The main advantage of this system consists in that the production of themodified orthophotograph is completely automatic and no errors can occursimilar to the errors that are regularly made by the operator inwatching a real optical model and performing the necessary Z-shiftaccordingly.

It is easy to expand the system described above in connection with FIG.12 so that a true or unmodified orthophotograph is producedsimultaneously with the modified orthophotograph. For this purpose, athird cathode ray tube provided with another film at its screen isconnected to the system with its Y-deflection coils coupled parallel tothe respective inputs of the CRT 140, and with its intensity controlinput coupled to an amplifier similar to the amplifier 142 and whichreceives its input from the other one 122 of the photosensitive cells.The X-deflection coils of the third CRT are connected to a separateamplifier which is similar to the amplifier 146 and is directly coupledto the X-defiection signal generator 138.

APPLICATION OF THE MODIFIED ORTHOPHOTOGRAPH (nos. 10 and 11) FIG. 10shows a chart 97 with two similar images 98, 99 disposed in a fixedside-by-side relationship, the left hand image 98 being an unmodifiedorthophotograph as it may have been produced on the prior art deviceaccording to FIG. 1, or on the right hand film 30a of the apparatusshown in FIG. 8. In the terrain depicted in this unmodifiedorthophotograph the numeral a designates a shore-line, to the right ofwhich a part of the sea can be seen. The terrain to the left of theshoreline 100a may be hilly, 10314 designating the ridge of a chain ofmountains extending along the shore. At 101a a buoy and at 102a anobservation post located above sea-level may be seen. The unmodifiedorthophotograph represents a photographic map with every detail in itstrue horizontal position. By taking the scale into consideration anyhorizontal distance can easily be measured in this orthophotograph.

The right hand image 99 on the chart 97 is a modified orthophotographwhich forms a stereo-partner to the unmodified orthophotograph in theimage 98 and which may have been produced on the device according toFIG. 4, 6 or 12 or on the left hand film 30m of the apparatus shown inFIG. 8. Assuming that the horizontal displacement of the film inproducing the modified orthophotograph has been performed in thedirection of the abscissa of the images 98, 99, i.e. that theX-direction defined above is parallel to the abscissa, the terraindepicted in the image 99 is distorted according to differences in theelevation of the terrain only in respect of distances along thedirection of the abscissa. As a result the distance M is unequal to thedistance U, because the buoy and the observation post are at differentaltitudes, whereas any distance in the image 99 in a direction parallelto the ordinate, i.e. parallel to the Y-direction, is the same as in theimage 98. Assuming furthermore that the datum plane of the terrain isselected to be horizontal, preferably coincident with the sea level, theshape of the shore-line 100m in the image 99 is exactly the same as theshape of the shore-line 10014 in the image 98 and, equally, the positionof the buoy 101m with respect to the shore-line 100m in the image 99 isthe same as the position of the buoy 10114 with respect to theshore-line 10014 in the image 98.

According to the differences in height, the line 103m representing theridge of the chain of mountains in the image 99 has a shape and aposition different from the shape and the position of the line 10314 inthe image 98. Similarly, the observation post 102m in the image 99 has adifferent position than in the image 98.

When viewing the unmodified orthophotograph in the image 98stereoscopically together with the modified orthophotograph in the image99 by means of an ordinary stereoscope, the formation of the terrain tothe left of the shore-line will appear three dimensional. Verticaldistances, either elevations relative to the datum level or spotheights, may be measured easily and quickly by means of a stereoscopicdepth scale printed on an overlay 106. The unmodified and the modifiedorthophotographs have to be disposed in a fixed relationship such as onthe chart 97, and the depth scale may comprise two groups of marks 107a,107m spaced apart in the X-direction with the group 107a appearing overthe unmodified orthophotograph and the other group 107m appearing overthe modified orthophotograph. The distance between adjacent marks of onegroup is different from the corresponding distance in the other group.Each mark of any group corresponds to a mark of the other group, andeach pair of marks so formed will produce a fused image that will appearvertically offset from the datum plane when viewed stereoscopically, therespective pairs thus produce apparent images at spaced levels abovesuch plane. By properly annotating the marks of each of the groups 107u,107m and by comparing the apparent height of a detail in question withthe vertical scale so-formed, the elevation of the detail may be readoff directly.

The combination of a true orthophotograph and its modifiedstereo-partner makes it particularly easy to visualize the line of sightbetween two points. In the example according to FIG. 10, it may be ofinterest to determine whether the buoy llu can be seen from theobservation post 102a. For this purpose a straight line 104a connectingthe observation post 102a and the buoy l0lu can either be drawn directlyon the image 98 or on a transparent sheet placed thereon and a similarstraight line 104m connecting the observation post 102m and the buoy101m formed on the image 99. Since the position of the observation post102m in the modified orthophotograph in the image 99 is different fromthe position of the observation post l02u in the unmodifiedorthophotograph in the image 98, the lines 104m and 104u will havedifferent angles with respect to the X-direction, and therefore, thefused image of these lines under the stereoscope will appear inclined tothe horizontal plane of the sea. Assuming that the terrain depicted inthe two orthophotographs has the profile 105 shown in FIG. 11, the fusedline of sight will ap pear to penetrate the chain of mountains, whichfact indicates that the buoy l0lu cannot be seen from the observationpost 10214.

The complete area of view from a selected point situated in or above theterrain can similarly be mapped by pivoting two lines from thecorresponding representations of such selected point in the unmodifiedand in the modified orthophotograph. The following examples arerepresentative of the variety of applications for which this type ofmapping may be used:

1. for military purposes, it is an obvious advantage to be able toproduce quickly an accurate map of the field of view of an observer (ora radar set) from any friendly or enemy vantage point. Again, astereoscopic model of the trajectory of a shell could be viewed in thesame way to determine whether it will clear a hilltop;

2. for surveyors, the method will be valuable for preliminary locationof triangulation or trilateration points. Even if there has beeninsufiicient ground control to establish a datum plane in the terrainmodel, the relative orientation process will assure that the fields ofview that are determined by this method will be correct over each model;

3. the field of view from a forest fire rangers tower can be mappedbefore the tower is built, without visiting the site;

4. the location of microwave relay towers depending upon line-of-sightpropagation will be assisted by this method.

The constant vertical and horizontal scale of theorthophotostereo-partner combination renders it possible to devicespecial tools for slope measurement which will require no calculation. Aset of stereoscopic cones of varying angle can be constructed and theslope of a specific portion of terrain can easily be measured bycomparison with these cones.

As mentioned above, true orthophotographs may be joined along any linesof common terrain points to fonn a mosaic with constant horizontalscale. Modified stereo-partners may also be made into a continuousmosaic which can be viewed stereoscopically with the first mosaic. Theresulting optical model of the terrain is continuous and constant inscale both vertically and horizontally. To make stereoscopic mosaics,the scan direction will preferably be parallel to a constant geographicdirection through the whole mosaic, this direction also being thedirection of the flight lines of the aircraft.

The above described method and apparatus is based on the assumption thatthe flying height of the aeroplane from which the aerial photographs aretaken, is low enough to consider the reference base as a plane. This,however, is no longer true if the photographs are taken from a spacecraft and comprise a considerable portion of the entire surface of theearth or another spherical celestial body. In this latter case, the areaof the film can remain fiat and the horizontal displacement during theproduction of the modified orthophotograph can be replaced by a rotationor any suitable motion of the film that creates a map-likerepresentation of the photographic detail. In the case of photographyfrom a space craft that approaches the celestial body perpendicularly tothe body surface, the two photographs may be taken along a common axis(i.e. the direction of approach), and at different distances from thesurface. Such a pair of photographs is considered as a stereopair"within the meaning of this term as used in the following claims. Exceptfor the area at the nadir point below the craft, the two photographswill show parallaxes of the details due to their elevation. Whenproperly brought to a common scale, these photographs thus form astereo-pair.

In the above description only the common near-vertical aerialphotographs have been considered, although the invention may be appliedto non-topographic stereoscopic photog raphy.

I claim:

1. A device for producing a modified orthophotograph, comprising a.means for obtaining from two photographs of an object forming astereo-pair with each other the X-, Y- and Z- coordinates andinformation concerning the image of each detail of said object,including means for generating data corresponding to a trueorthophotograph of said object,

b. means for modifying said data by varying the value of eachX-coordinate of said data by an amount depending on the value of thecorresponding said Zcoordinate, and

c. means forming said modified orthophotograph from said modified data.

2. A device according to claim 1, wherein said data obtaining andgenerating means (a) comprises i. means for generating two beams oflight having a common intersection point, each of said beams passingthrough one of said photographs,

ii. means for placing said beam intersection point at successive X- andY-coordinates of a plane, and

iii. means for varying the Z-coordinate of said intersection point alonga direction perpendicular to said plane;

and wherein said forming means (c) comprise iv. a photo-sensitive film,

v. a source of light and means for varying the intensity of said sourceof light in accordance with the intensity of one of said beams afterpassing through the respective one of said photographs, and

vi. means for'placing said source of light relative to said film at eachof said modified X-coordinates coordinates and each of saidY-coordinates of a second plane parallel to said film.

3. A deviceaccording to claim 1, wherein said data obtaining andgenerating means (a) comprise i. two projectors supported side-by-sidefor superimposing said photographs to create a three-dimensional opticalmodel of said object, and

ii. a base plate supporting said film and having means for shifting saidbase plate relative to said projectors with a first motion along a firstpath perpendicular to said film;

wherein said modifying means (b) comprises iii. means for displacingsaid base plate with a second motion along a second path intersectingsaid first path, and

iv. means interconnecting said shifting means and said displacing meansto interrelate said first and second motions so that the amount of saidsecond motion is dependent on the amount of said first motion; and

wherein said forming means (c) comprise means for successively exposingportions of said film to light projecting the image of one of saidphotographs.

4. A device according to claim 3 wherein said exposing means comprisesan aperture and means for moving said aperture in adjacent tracesparallel to a scanning direction across said film.

5. A device according to claim 4 wherein said means for moving saidaperture comprises carriage means provided with said aperture andmovable in said scanning direction, and guide means for said carriage,movable in an indexing direction perpendicular to said scanningdirection.

6. A device according to claim 3 wherein said interconnecting meanscomprises two parts, said parts comprising cam means and follower meansfor said cam means, one of said parts being fixed relative to saidprojectors, and the other of said parts being fixed relative to saidbase plate.

'7. A device according to claim 6 wherein said cam means comprises astraight cam rail disposed parallel to a plane containing said first andsecond paths and inclined to said film.

8. A device according to claim 6, wherein said cam means comprises a camrail disposed parallel to a plane containing said first and second pathsand logarithmically curved.

9. A device according to claim 8, including linear cam means interposedbetween said logarithmic cam means and either said projectors or saidbase plate, said linear cam means including means for adjusting theratio thereof.

10. A device according to claim 3, further comprising an optical systemfor viewing said optical model, said system having a binocular fixedrelative to said projectors.

11. A device according to claim 3, further comprising,

d. a second base plate for supporting a second photosensitive film andsecond means for successively exposing portions of said second film;

e. means for projecting onto said second film a third image identical tothe image of the stereo-mate of said one photograph; and

f. coupling means for transferring the shift of the first base platealong said first path to said second base plate.

12. A device according to claim 11 wherein said means for projectingsaid third image comprises a third projector for holding a furtherphotograph identical to said stereo-mate.

13. A device according to claim 11, wherein said coupling meanscomprises a mounting plate supporting said first and second base plates.

14. A device according to claim 11, wherein said second exposing meansare coupled to the first mentioned exposure means.

15. A device according to claim 1, wherein said data obtaining andgenerating means (a) comprise i. a first cathode ray tube having ascreen, and means for shifting said first CRT in a directionperpendicular to said screen,

ii. two photo-sensitive cells, each of said cells being disposed toreceive a beam emanating from a spot of light on said screen and passingthrough the respective one of said photographs, and

iii. means for correlating the signals generated by said cells andproviding a signal for driving said shifting means; and

wherein said forming means (c) comprise iv. a second cathode ray tubehaving means for controlling the intensity of the cathode ray of saidsecond CRT, said controlling means being connected to one of said cells,and

v. a photo-sensitive film disposed to be exposed by the cathode ray ofsaid second CRT.

16. A device according to claim 15 wherein said data obtaining andgenerating means furthermore comprise an X- deflection signal generatorand a Y-defiection signal generator, wherein said first CRT comprises afirst X-deflection system and a first Y-deflection system and saidsecond CRT comprises a second X-defiection system and a second Y-deflection system, said first and second X-deflection systems beingconnected to said X-deflection signal generator and said first andsecond Y-deflection systems being connected to said Y-deflection signalgenerator.

17. A device according to claim 16, wherein said modifying meanscomprise a modifier having two signal inputs and one signal output, oneof said inputs being connected to said correlating means and the otherof said inputs to said X-deflection signal generator, and said outputbeing connected to said second X-deflection system.

18. A device for producing a modified orthophotograph, comprising a.means for obtaining from two photographs of an object forming astereo-pair with each other the X-, Y- and Z- coordinates andinformation concerning the image of each detail of said object,including means for generating data corresponding to a trueorthophotograph of said object,

b. means for modifying said data by introducing into each X-coordinateof said data a change related to the corresponding said Z-coordinate inaccordance with a predetermined function, and

0. means for forming said modified orthophotograph from said modifieddata, I

d. wherein said function comprises a logarithmic component and a linearcomponent.

19. A device for producing a modified orthophotograph,

comprising a. means for obtaining from two photographs of an objectforming a stereo-pair with each other the X-, Y- and Z- coordinates andinformation concerning the image of each detail of said object,including means for generating data corresponding to a trueorthophotograph of said object; I b. means for modifying said data byintroducing into each X-coordinate of said data a change related to thecorresponding said Z-coordinate in accordance with a predeterminedfunction; c. means for forming said modified orthophotograph from saidmodified data; d. wherein said data obtaining and generating means (a)comprise i. two projectors supported side-by-side for superimposing saidphotographs to create a three-dimensional optical model of said object,and

ii. a base plate supporting said film and having means for shifting saidbase plate relative to said projectors with a first motion along a firstpath perpendicular to said film;

e. wherein said modifying means (b) comprise iii. means for displacingsaid base plate with a second motion along a second path intersectingsaid first path, and

cessively exposing portions of said film to light projecting the imageof one of said photographs; and g. wherein said interconnecting meanscomprise v. logarithmic cam means and follower means for said cam means,one of said cam and follower means being connected to said projectors,and the other of said cam and follower means being connected to saidbase plate, and

vi. linear cam means interposed between said logarithmic cam means andeither said projectors or said base plate,

said linear cam means including means for adjusting the ratio thereof.

*li i I

1. A device for producing a modified orthophotograph, comprising a.means for obtaining from two photographs of an object forming astereo-pair with each other the X-, Y- and Z-coordinates and informationconcerning the image of each detail of said object, including means forgenerating data corresponding to a true orthophotograph of said object,b. means for modifying said data by varying the value of eachXcoordinate of said data by an amount depending on the value of thecorresponding said Z-coordinate, and c. means forming said modifiedorthophotograph from said modified data.
 2. A device according to claim1, wherein said data obtaining and generating means (a) comprises i.means for generating two beams of light having a common intersectionpoint, each of said beams passing through one of said photographs, ii.means for placing said beam intersection point at successive X- andY-coordinates of a plane, and iii. means for varying the Z-coordinate ofsaid intersection point along a direction perpendicular to said plane;and wherein said forming means (c) comprise iv. a photo-sensitive film,v. a source of light and means for varying the intensity of said sourceof light in accordance with the intensity of one of said beams afterpassing through the respective one of said photographs, and vi. meansfor placing said source of light relative to said film at each of saidmodified X-coordinates coordinates anD each of said Y-coordinates of asecond plane parallel to said film.
 3. A device according to claim 1,wherein said data obtaining and generating means (a) comprise i. twoprojectors supported side-by-side for superimposing said photographs tocreate a three-dimensional optical model of said object, and ii. a baseplate supporting said film and having means for shifting said base platerelative to said projectors with a first motion along a first pathperpendicular to said film; wherein said modifying means (b) comprisesiii. means for displacing said base plate with a second motion along asecond path intersecting said first path, and iv. means interconnectingsaid shifting means and said displacing means to interrelate said firstand second motions so that the amount of said second motion is dependenton the amount of said first motion; and wherein said forming means (c)comprise means for successively exposing portions of said film to lightprojecting the image of one of said photographs.
 4. A device accordingto claim 3 wherein said exposing means comprises an aperture and meansfor moving said aperture in adjacent traces parallel to a scanningdirection across said film.
 5. A device according to claim 4 whereinsaid means for moving said aperture comprises carriage means providedwith said aperture and movable in said scanning direction, and guidemeans for said carriage, movable in an indexing direction perpendicularto said scanning direction.
 6. A device according to claim 3 whereinsaid interconnecting means comprises two parts, said parts comprisingcam means and follower means for said cam means, one of said parts beingfixed relative to said projectors, and the other of said parts beingfixed relative to said base plate.
 7. A device according to claim 6wherein said cam means comprises a straight cam rail disposed parallelto a plane containing said first and second paths and inclined to saidfilm.
 8. A device according to claim 6, wherein said cam means comprisesa cam rail disposed parallel to a plane containing said first and secondpaths and logarithmically curved.
 9. A device according to claim 8,including linear cam means interposed between said logarithmic cam meansand either said projectors or said base plate, said linear cam meansincluding means for adjusting the ratio thereof.
 10. A device accordingto claim 3, further comprising an optical system for viewing saidoptical model, said system having a binocular fixed relative to saidprojectors.
 11. A device according to claim 3, further comprising, d. asecond base plate for supporting a second photosensitive film and secondmeans for successively exposing portions of said second film; e. meansfor projecting onto said second film a third image identical to theimage of the stereo-mate of said one photograph; and f. coupling meansfor transferring the shift of the first base plate along said first pathto said second base plate.
 12. A device according to claim 11 whereinsaid means for projecting said third image comprises a third projectorfor holding a further photograph identical to said stereo-mate.
 13. Adevice according to claim 11, wherein said coupling means comprises amounting plate supporting said first and second base plates.
 14. Adevice according to claim 11, wherein said second exposing means arecoupled to the first mentioned exposure means.
 15. A device according toclaim 1, wherein said data obtaining and generating means (a) comprisei. a first cathode ray tube having a screen, and means for shifting saidfirst CRT in a direction perpendicular to said screen, ii. twophoto-sensitive cells, each of said cells being disposed to receive abeam emanating from a spot of light on said screen and passing throughthe respective one of said photographs, and iii. means for correlatingthe signals generated by said cells and providing a signal for drivingsaid shifting means; and wherein said Forming means (c) comprise iv. asecond cathode ray tube having means for controlling the intensity ofthe cathode ray of said second CRT, said controlling means beingconnected to one of said cells, and v. a photo-sensitive film disposedto be exposed by the cathode ray of said second CRT.
 16. A deviceaccording to claim 15 wherein said data obtaining and generating meansfurthermore comprise an X-deflection signal generator and a Y-deflectionsignal generator, wherein said first CRT comprises a first X-deflectionsystem and a first Y-deflection system and said second CRT comprises asecond X-deflection system and a second Y-deflection system, said firstand second X-deflection systems being connected to said X-deflectionsignal generator and said first and second Y-deflection systems beingconnected to said Y-deflection signal generator.
 17. A device accordingto claim 16, wherein said modifying means comprise a modifier having twosignal inputs and one signal output, one of said inputs being connectedto said correlating means and the other of said inputs to saidX-deflection signal generator, and said output being connected to saidsecond X-deflection system.
 18. A device for producing a modifiedorthophotograph, comprising a. means for obtaining from two photographsof an object forming a stereo-pair with each other the X-, Y- andZ-coordinates and information concerning the image of each detail ofsaid object, including means for generating data corresponding to a trueorthophotograph of said object, b. means for modifying said data byintroducing into each X-coordinate of said data a change related to thecorresponding said Z-coordinate in accordance with a predeterminedfunction, and c. means for forming said modified orthophotograph fromsaid modified data, d. wherein said function comprises a logarithmiccomponent and a linear component.
 19. A device for producing a modifiedorthophotograph, comprising a. means for obtaining from two photographsof an object forming a stereo-pair with each other the X-, Y- andZ-coordinates and information concerning the image of each detail ofsaid object, including means for generating data corresponding to a trueorthophotograph of said object; b. means for modifying said data byintroducing into each X-coordinate of said data a change related to thecorresponding said Z-coordinate in accordance with a predeterminedfunction; c. means for forming said modified orthophotograph from saidmodified data; d. wherein said data obtaining and generating means (a)comprise i. two projectors supported side-by-side for superimposing saidphotographs to create a three-dimensional optical model of said object,and ii. a base plate supporting said film and having means for shiftingsaid base plate relative to said projectors with a first motion along afirst path perpendicular to said film; e. wherein said modifying means(b) comprise iii. means for displacing said base plate with a secondmotion along a second path intersecting said first path, and iv. meansinterconnecting said shifting means and said displacing means tointerrelate said first and second motions in accordance with saidfunction; f. wherein said forming means (c) comprise means forsuccessively exposing portions of said film to light projecting theimage of one of said photographs; and g. wherein said interconnectingmeans comprise v. logarithmic cam means and follower means for said cammeans, one of said cam and follower means being connected to saidprojectors, and the other of said cam and follower means being connectedto said base plate, and vi. linear cam means interposed between saidlogarithmic cam means and either said projectors or said base plate,said linear cam means including means for adjusting the ratio thereof.